1. Field of the Invention
The present invention relates to a cooling roll, ribbon-shaped magnetic materials, magnetic powders and bonded magnets. More specifically, the present invention relates to a cooling roll, a ribbon-shaped magnetic material formed by using the cooling roll, a magnetic powder formed from the magnetic material and a bonded magnet manufactured using the magnetic powder.
2. Description of the Prior Art
Rare-earth magnetic materials formed from alloys containing rare-earth elements have high magnetic properties. Therefore, when they are used for magnetic materials for motors, for example, the motors can exhibit high performance.
Such magnetic materials are manufactured by the quenching method using a melt spinning apparatus, for example. Hereinbelow, explanation will be made with regard to the manufacturing method using the melt spinning apparatus.
FIG. 20 is a sectional side view which shows the situation caused at or around a colliding section of a molten alloy with a cooling roll in the conventional melt spinning apparatus which manufactures a magnetic material using a single roll method.
As shown in this figure, in the conventional method, a magnetic material made of a predetermined alloy composition (hereinafter, referred to as “alloy”) is melt and such a molten alloy 60 is injected from a nozzle (not shown in the drawing) so as to be collided with a circumferential surface 530 of a cooling roll 500 which is rotating relative to the nozzle in the direction indicated by the arrow A in FIG. 20. The alloy which is collided with the circumferential surface 530 is quenched and then solidified, thereby producing a ribbon-shaped alloy in a continuous manner. This ribbon-shaped alloy is called as a melt spun ribbon. Since the melt spun ribbon was quenched in a rapid cooling rate, its microstructure has a structure composed of an amorphous phase or a microcrystalline phase, so that it can exhibit excellent magnetic properties as it is or by subjecting it to a heating treatment. In this regard, it is to be noted that the dotted line in FIG. 20 indicates a solidification interface of the molten alloy 60.
The rare-earth elements are liable to oxidize. When they are oxidized, the magnetic properties thereof tend to be lowered. Therefore, normally, the manufacturing of the melt spun ribbon is carried out under an inert gas atmosphere. However, this causes the case that gas enters between the circumferential surface 530 and the puddle 70 of the molten alloy 60, which results in formation of dimples (depressions) 9 in the roll contact surface 810 of the melt spun ribbon 80 (that is, the surface of the melt spun ribbon which is in contact with the circumferential surface 530 of the cooling roll 500). This tendency becomes prominent as the peripheral velocity of the cooling roll 500 becomes large, and in such a case the area of the formed dimples becomes also larger.
In the case where such dimples 9 (especially, huge dimples) are formed, the molten alloy 60 can not sufficiently contact with the circumferential surface 530 of the cooling roll 500 at the locations of the dimples due to the existence of the entered gas, so that the cooling rate is lowered to prevent rapid solidification. As a result, at portions of the melt spun ribbon where such dimples are formed, the crystal grain size of the alloy becomes coarse, which results in lowered magnetic properties.
Magnetic powder obtained by milling such a melt spun ribbon having the portions of the lowered magnetic properties has larger dispersion or variation in its magnetic properties. Therefore, bonded magnet formed from such magnetic powder can have only poor magnetic properties, and corrosion resistance thereof is also low.